The present invention relates generally to an anti-skid brake control system for an automatic brake system, which holds brake pressure to be applied to each wheel at approximately a lock pressure at which the vehicle wheel locks to cause skidding. More specifically, the invention relates to a quick take-up function for buildng up brake pressure close to the lock pressure at a relatively early timing.
As is well known, optimum braking characteristics are obtained when braking pressure or force can be adjusted that the peripheral speed of the wheels during braking is held to a given ratio, e.g. about 80% to 85%, of the vehicle speed. This practice is believed to be particularly effective when road conditions and other factors are taken into consideration. On the other hand, if wheel speed is held to a ratio to the vehicle speed higher than the above-mentioned optimal ratio, e.g., 80% to 85%, braking distance may be prolong due to a lack of braking pressure. On the other hand, if the braking pressure is so adjusted at to hold the wheel speed to a ratio with respect the vehicle speed less than the aforementioned optimal ratio, the vehicle wheels may lock and skid, resulting in an unnecessarily long braking distance due to reduced traction. In practice, it is very difficult to precisely adjust the braking pressure so the the wheel speed is held to the given optimal ratio to the vehicle speed.
In the practical anti-skid brake control operation, braking pressure in one of more wheel cylinders is adjusted by cyclically increasing and decreasing the braking pressure in the wheel cylinder. The anti-skid control system generally decreases braking pressure when the wheel deceleration value becomes less than a given deceleration threshold, which is so chosen as to prevent the wheel from skidding, and increases braking pressure when the wheel acceleration value is greater than a given acceleration threshold. In this conventional anti-skid brake control procedure, wheel speed does not remain in an optimal relationship to the vehicle speed for a satisfactorily long period of time.
In order to improve performance, U.S. Pat. No. 3,637,264, issued on Jan. 25, 1972 to Leiber et al discloses an Antilocking Control for Pressure Actuated Brakes. The pressure of the brake-actuating fluid in an antilocking brake control system is varied by pulsing the control valve or valves for durations which are varied to be a greater of lesser extent than the period of that limiting frequency above which the brake system cannot respond. In the former case, a rapid increase in fluid pressure or a rapid decrease in fluid pressure occurs, whereas in the latter case, a less rapid average or net increase or decrease occurs in the fluid pressure to which the brake system responds. These conditions are controlled in dependence on the rotational behavior of the vehicle wheel or wheels and more especially in dependence or predetermined changes in angular velocity of the wheel. Moreover, either variation in pulse duration at a fixed frequency or variation in frequency at a fixed pulse duration may be effected during high-frequency pulsing so as further to alter the net increase or decrease in fluid pressure. This further alternation is effected as a function of time from the beginning of the high-frequency pulsing.
In addition, Published Japanese Patent Application (Tokkai) No. Showa 51-89096, published on Aug. 4, 1976 discloses a system to the above. The fluid pressure in the wheel cylinder is increased in a stepwise manner. Duration of increase of the fluid pressure is adjusted in accordance with the rate of increase of the fluid pressure in one or more preceding steps.
In such conventional anti-skid brake control system, hunting or switching of the control mode between an application mode in which brake pressure increases and a subsequent hold mode in which brake pressure is held constant at the increased level tends to occur when friction on the road surface is relatively high. This occurs because of a relatively quick recovery of wheel speed during the hold mode. Hunting between the application mode and subsequent hold mode may degrade riding comfort and, as well, prolongate braking distance due to a rather long period of time for reaching a lock pressure where braking efficiency become approximately optimum.